Plasma-arc torches, also known as electric arc torches, are commonly used for cutting, welding and spray bonding and operate by directing a plasma consisting of ionized gas particles toward a workpiece. In operation of typical plasma-arc torches, a gas to be ionized is supplied to the front end of the torch and flows past an electrode before exiting through an orifice in the torch tip. The electrode has a relatively negative potential and operates as a cathode. The torch tip which is adjacent to the end of the electrode at the front end of the torch, constitutes a relatively positive potential anode. When a sufficiently high voltage is applied to the electrode, an arc is caused to jump the gap between the electrode and the torch tip, thereby heating the gas and causing it to ionize. The ionized gas in the gap is blown out of the torch and appears as a flame that extends externally off the tip. As the torch head or front end is brought down towards the workpiece, the arc jumps or transfers between the electrode and the workpiece because the impedance of the workpiece to ground is lower than the impedance of the torch tip to ground. During this "transferred arc" operation, the workpiece itself serves as the anode.
Plasma-arc torches may be found in both "non-contact start" and "contact start" varieties. In non-contact start torches, the tip and electrode are typically maintained at a fixed physical separation in the torch head. When a high voltage is applied to the electrode (relative to the tip), a pilot arc is established therebetween. As mentioned above, when the torch head is moved toward the workpiece, the arc transfers to the workpiece. By way of contrast, in contact start torches the tip and/or electrode move relative to each other (e.g., along a longitudinal axis of the electrode). For example, the tip may be biased by a spring such that a clearance distance is maintained between the tip and electrode. In order to initiate a pilot arc, the torch operator places the torch head in contact with the work piece with sufficient force to cause the tip to compress the biasing spring thus allowing the tip and electrode to make electrical contact. This electrical contact enables establishment of a pilot arc. As the operator moves the torch head away from the workpiece, the arc is transferred to the workpiece.
Typically, the electrode and torch tip are consumable items, that is they eventually need replacement. As such, the torch head allows for removal and replacement of both the tip and electrode. Further, it is undesirable to operate a plasma-arc torch when either the tip or electrode is not properly installed. Therefore, plasma-arc torches can benefit from parts-in-place detection circuits for determining whether the tip and/or electrode are properly installed. In non-contact start torches these detection circuits may include a circuit for sensing tip installation, electrode installation, and that the tip and electrode are not in electrical contact. However, the same parts-in-place system used for a non-contact start torch may not work for contact start torches because in a contact start torch, electrical contact between the tip and electrode is necessary during pilot arc initiation.
Moreover, conventional plasma torch parts-in-place detection systems typically use additional electrical or mechanical means, such as contact pins, microswitches and the like, within the torch head or torch leads. This, of course, increases the cost and complicates the design and manufacture of the torch and lead assembly. Further, conventional plasma torches require a relatively high open circuit voltage (e.g., -300 VDC) at the electrode in order to ionize the flowing gas. If the tip were not installed, the electrode would be exposed. An exposed electrode with such a high voltage potential presents an undesirable condition.
There is a need, therefore, for a low cost parts-in-place detection system for use with a contact start plasma-arc torch that requires no additional electrical or mechanical means or wiring within the torch head or torch leads. There is a further need to accomplish the parts-in-place function using a relatively low open circuit voltage rather than the full output voltage otherwise used for normal torch operations.